Today, in general only two different cooling systems are used for submersible pumps: closed jacket cooling system and open jacket cooling system.
The first system, closed jacket cooling, uses the same pumped liquid to cool the motor thanks to a cooling jacket installed around the cast iron casing, as schematically shown in FIG. 1. In this case the liquid must not contain big solid parts to prevent the cooling circuit flow to be reduced or clogged. The mechanical seals are arranged in an oil chamber. The closed jacket cooling system does not require external water, which can be expensive and sometimes not easily available in the sump, but at the same time, since it uses the dirty liquid for cooling, has major drawbacks. As mentioned before the disadvantage is the use of the same pumped liquid. In pumps intended mainly for sewage, or any other dirty water, the use of the pumped water for cooling can create problems and requires frequent maintenance and cleaning of the cooling circuit.
The second system, shown schematically in FIG. 2, open jacket cooling, uses external water to cool the motor. It can be a closed circuit (with a well dimensioned water reservoir) or the water coming out from the motor, after having cooled it, can be released in the sump. The open jacket cooling system has the advantage that it avoids using the dirty liquid for cooling, but it does require fresh external water for cooling, which can be expensive and sometimes not easily available on site.
These systems are still used by most pump manufacturers today.
In the prior art, a closed loop cooling system with dry motor (i.e. no oil in the motor) and without any additional cooling circulator has been proposed. This system does not need external water or frequent maintenance.
One disadvantage of this prior art system is that, in cooling installed system version, the pumps comes in contact with water not only in the cooling system, but also in the mechanical seals chamber. This last characteristic is the main disadvantage of this prior art system. In submersible pumps it is very common to have the mechanical seals in a separate oil chamber and provide a leakage detector therein. The leakage detector is basically a probe that can detect if the oil is contaminated by water.
The purpose of mechanical seals is to prevent water from reaching the motor. The reason why most manufacturers employ two mechanical seals is because when there is a failure of the first one, the pump does not need to be stopped. The leakage detector gives alarm, and the second seal still prevents the water from going into the motor chamber.
In the above prior art pumps with closed loop cooling system, the oil chamber is filled with glycol, the same as the chamber of the cooling system. In this prior art, a probe can only be placed in the motor to detect when water reaches the motor, i.e. once both mechanical seals have failed. At the same time when this incident happens, it means that the sewage water has entered the first mechanical seals, may be pumped in the cooling system for days and may come in direct contact with the second mechanical seals, since this condition cannot be detected. Only when the second seal fails and the sewage water starts to reach the motor chamber, the water detector installed there can finally switch off the pump.
Hence in this prior art there is no early alarm after which the pump can still run and maintenance be organized. Rather in the system according to that prior art, when there is an alarm, it is too late and the pump has to be switched off immediately.
In that prior art the cooling chamber maintenance will be very difficult due to the sewage liquid stacked around the motor and between the mechanical seals.
Another disadvantage of this prior art is also the normal maintenance. Normally when a pump is serviced, the operator checks the oil level in the mechanical seals. In this configuration the pump cooling liquid, which is the same as for the mechanical seals, has to be emptied completely and refilled.
A further disadvantage is also the poorer characteristic of glycol compared to specifically designed oil for mechanical seals.
A further prior art uses an active circulator to keep the cooling liquid moving. However, the additional electrical external motor required in this case has further disadvantages in that it increases maintenance, requires additional cables to the pump, and generally increases the chances of mechanical failure.
It is therefore an object of the present invention to overcome or alleviate at least some of the disadvantageous of the known cooling systems for submersible pumps.